1. Field of the Invention
The present invention generally relates to an electrolyte tank and, more specifically, to an electrolyte tank used for an electrolyte circulating type battery in which an electrolyte is caused to flow and circulate between electrodes for charging/discharging on the electrodes. The present invention further relates to a method of manufacturing such an electrolyte tank.
2. Description of the Background Art
Various new types of batteries have been developed as batteries for storing power to substitute for pumped storage power generation. Among such new types of batteries, a redox flow battery has been particularly attracting attention.
FIG. 8 is a schematic diagram of a redox flow battery as a representative example of the conventionally proposed electrolyte circulating type battery.
Referring to FIG. 8, a redox flow battery 1 includes a reaction cell 6, a positive electrolyte tank 2 and a negative electrolyte tank 3. Reaction cell 6 is partitioned by a diaphragm 4 formed of an ion exchange membrane or the like into two portions, one serving as a positive electrode cell 6a and the other serving as a negative electrode cell 6b. 
Positive electrode cell 6a accommodates a positive electrode 7 and negative electrode cell 6b accommodates a negative electrode 8.
Positive electrode cell 6a and positive electrolyte tank 2 are coupled by a positive electrolyte feeding tube 9 feeding the positive electrolyte to positive electrode cell 6a, and a positive electrolyte recovering tube 10 recovering the positive electrolyte from positive electrode cell 6a to positive electrolyte tank 2.
In positive electrolyte feeding tube 9, a pump 11 as positive electrolyte feeding and circulating means is provided, so as to allow circulation of the positive electrolyte between positive electrode cell 6a and positive electrolyte tank 2.
Negative electrode cell 6a and negative electrolyte tank 3 are coupled by a negative electrolyte feeding tube 12 feeding the negative electrolyte from negative electrolyte tank 3 to negative electrode cell 6b and a negative electrolyte recovering tube recovering the negative electrolyte from negative electrode cell 6b to negative electrolyte tank 3.
Further, in negative electrolyte feeding tube 12, a pump 14 as negative electrolyte feeding and circulating means is provided, allowing circulation of the negative electrolyte between negative electrode cell 6b and negative electrolyte tank 3.
In positive electrolyte tank 2, positive electrolyte as reactive liquid is stored, and in negative electrolyte tank 3, negative electrolyte as reactive liquid is stored.
As the positive electrolyte, aqueous solution of ions such as Fe ions of variable valence is used, and as the negative electrolyte, aqueous solution of ions such as chromium ions with variable valence is used.
A hydrochloric acid aqueous solution containing positive active substance Fe3+/Fe2+may be used as the positive electrolyte, and a hydrochloric acid aqueous solution containing negative active substance Cr2+/Cr3+ may be used as the negative electrolyte, for example.
In redox flow battery 1 using such electrolytes, at the time of charging, the hydrochloric acid aqueous solution containing Cr3+ ions stored in negative electrolyte tank 3 is fed to negative electrode cell 6b by means of pump 14, electrons are received at negative electrode 8 so that ions are reduced to Cr2+ ions, and recovered to negative electrolyte tank 3.
The hydrochloric acid aqueous solution containing Fe2+ ions stored in positive electrolyte tank 2 is fed to positive electrode cell 6a by means of pump 11, electrons are emitted to an external circuitry at positive electrode 7, so that ions are oxidized to Fe3+ ions, and recovered to positive electrolyte tank 2.
At the time of discharging, the hydrofluoric acid aqueous solution containing Cr2+ ions stored in negative electrolyte tank 3 is fed to negative electrode cell 6b by means of pump 14, electrons are emitted to the external circuitry at negative electrode 8, so that ions are oxidized to Cr3+ ions and recovered to negative electrolyte tank 3.
The hydrochloric acid aqueous solution containing Fe3+ ions stored in positive electrolyte tank 2 is fed to positive electrode cell 6a by means of pump 11, electrons are received from the external circuitry so that ions are reduced to Fe2+ ions, and recovered to positive electrolyte tank 2.
In such a redox flow battery, the charging/discharging reactions at positive electrode 7 and negative electrode 8 are as follows.                     positive        ⁢                  xe2x80x83                ⁢        electrode        ⁢                  :                ⁢                  xe2x80x83                ⁢                  Fe                      3            +                              +      e        ⁢                  ⇄        discharge            charge        ⁢          Fe              2        +                                negative        ⁢                  xe2x80x83                ⁢        electrode        ⁢                  :                ⁢                  xe2x80x83                ⁢                  Cr                      2            +                              +        ⁢                  ⇄        discharge            charge        ⁢                  Cr                  3          +                    +      e      
Electromotive force of about 1V can be obtained by the above described charging/discharging reactions.
In the conventional electrolyte circulating type battery having the above described structure, electrolyte tanks 2 and 3 are formed as a box-shaped or cylindrical shaped container of metal or FRP with a chemical resistant resin layer provided inside the container. Accordingly, installation requires considerable labor comparable to a general construction work. Further, it has been necessary to secure a place for installation. Further, reliability has been low because of leakage of the electrolyte at a connecting portion of the material. Further, when there is a stress distorted slightly, the battery is prone to cracks, resulting in leakage of the electrolyte. Further, it has been difficult to make use of existing space.
Therefore, an object of the present invention is to provide an electrolyte tank of which moving is easy.
Another object of the present invention is to provide an electrolyte tank which allows free use of existing space.
A still further object of the present invention is to provide an electrolyte tank of which installation is simple.
A still further object of the present invention is to provide an electrolyte tank having extremely high reliability at the connecting portion.
A still further object of the present invention is to provide an electrolyte tank free of any influence of a distortion to some extent.
A still further object of the present invention is to provide a method of manufacturing such an electrolyte tank.
The electrolyte tank in accordance with the present invention is formed as a bag-shaped flexible container in which membrane having one, or two or more laminated layers of coated fabric provided by coating woven fabric of organic fiber with rubber or plastic, is connected to a shape of the bag.
In the electrolyte tank in accordance with the present invention, even when the woven fabric is not very strong, it is unnecessary to separately prepare extra reinforcing member or the like, if the container is filled with the electrolyte so that the container is brought into tight contact with the whole space of a reservoir of a building for example, to generate load of internal pressure.
It may be effective to manufacture a tank of such a three-dimensional shape that confirms to the accommodating space in advance. Considering reliability at the connecting portion of the membrane, however, it may be preferable that the tank is manufactured as an envelope-like bag body, the tank is bent to a prescribed shape and thereafter the liquid is poured into the bag, to enable effective use of the space, as in the case where the tank is formed in a shape corresponding to the accommodating space. If the space is wide and open, the tank stands by itself if the woven fabric is adapted to have sufficient strength to withstand the internal pressure. Therefore, the tank may be installed at any place without special reinforcing member.
Further, it is possible to provide a manhole allowing passage of an operator, in the membrane of the electrolyte tank in accordance with the present invention. This allows human access during manufacturing of the bag-shaped body or for inspection of the internal surface when the tank fails.
In order to prevent as much as possible degradation of insulation from the manhole portion, it is preferable that the outer surface of the manhole portion is entirely covered by a rubber or plastic sheet or rubber or plastic coated fabric. At the time of emergency, the manhole can be used by tearing the cover on the outer surface, and after use, the torn cover may be removed from the connecting portion and a new cover may be re-applied.
In the electrolyte tank of the present invention, a metal, rubber or plastic film may be provided covering the outer side of the flexible bag-shaped container. This improves insulation, liquid leakage property and air permeability of the container than when not covered by such a film. Further, when a material having gas permeability coefficient of at most 1xc3x9710xe2x88x9210 ccxc2x7cm/cm2xc2x7secxc2x7cmHg is selected as the rubber or plastic, air permeation into the tank can be suppressed with such a film thickness that rigidity of the film is sufficiently low, and therefore degradation of the electrolyte by oxidation can be prevented.
When a layer mainly consisting of water absorbing polymer is provided on a surface not in contact with the electrolyte of the flexible bag-shaped container, it is possible to stop leakage in a short period of time, even if the container should be damaged, causing leakage of the electrolyte.
As to the organic fiber of the woven fabric, any general fiber may be used. Considering the possibility that the electrolyte comes to be in contact with the organic fiber after long time of use, however, organic fiber formed of chemical resistant resin such as polyester, polyethylene, fluoroplastics or the like, which is not degraded by the component of the electrolyte, is desirable. In view of strength and cost, polyester is the most preferable material.
As the rubber mentioned above, natural rubber or synthetic rubber may be used. Use of a chemical resistant material such as chlorosulfonated polethylene, EPDM (ethylene-propylene-diene-methylene) rubber, butyl rubber or the like, which is strong against electrolyte, is desirable. This suppresses permeation of the electrolyte, and hence provides an electrolyte tank which maintains insulation and durability over a long period.
A thermoplastic elastomer, which has been attracting attention recently as one type of rubber may be used as the flexible material. From the same reason as described above, it is preferable to select a chemical resistant material such as a polyorefin type material.
Even when the material of the rubber is not selective, it is desired that organic peroxide is used as the crosslinking agent of the rubber, than sulfur used as the crosslinking agent. Organic peroxide has the advantage of higher crosslink density, so that it suppresses the rate of permeation of the electrolyte and improves mechanical strength. Therefore, even when the material is the same, one crosslinked by the organic peroxide exhibits superior chemical resistance.
As the aforementioned plastics, any plastics generally available may be used. From the same reason as described with respect to rubber, a chemical resistant material such as vinyl chloride type or polyorefin type material is preferred.
As to the structure of the woven fabric, the fabric may have general structure such as plain weave or basket weave. When the rubber with which the woven fabric is to be coated is of a special material and it is difficult to establish adhesion with the woven fabric, for example, reliability at the interface of adhesion between the woven fabric and the rubber will be extremely low. Therefore, it is preferred that the woven fabric has open weave, so that the coating rubber on the front and back surfaces of the woven fabric is bridged and integrated.
As the woven fabric, any fabric having any strength may be used dependent on the condition of use. When the flexible bag-shaped container is to stand by itself, for example, the strength both in the warp and weft directions should be at least 400 kgf/in, taking into account the safety factor. Though it is possible to use a material having lower strength, durability is questionable when the container should stand safe by itself.
When the electrolyte tank of the present invention is formed as a rubber tank, it is possible, as in the conventional product formed of rubber coated fabrics, to joint the membrane to the shape of a bag in the unvulcanized state, and thereafter to vulcanize or crosslink the material by applying heat and pressure entirely. In order to nullify defect in the membrane, which may be the cause of lower insulation, however, it is desirable that the membrane is vulcanized and crosslinked before jointing work independently, and thereafter the vulcanized membrane is press- jointed. The press jointing may be performed using an adhesive, after physically roughening the surface of the vulcanized rubber as in the prior art. When the reliability of the jointing portion is considered, however, it is desired that press jointing is performed with unvulcanized rubber interposed. At this time, the unvulcanized rubber is vulvanized and integrated with the membrane.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.